Phenytoin pharmacokinetics is well known for normal body composition. These change with increasing obesity; i.e. distribution volume is increased, elimination half-life is prolonged, but total clearance is unchanged. Subj. with greater than average body wt., but body fat content below 10% represent a sub-group that has not been studied. The purpose is to determine if body composition (% fat + % lean tissue) may be a better predictor of phenytoin dose requirements than total body weight. This may apply to drugs in general.